While liquid crystal compositions are widely employed as materials for display devices, almost all currently used liquid crystal display devices employ Twisted Nematic (TN) type display mode which uses nematic phase. TN type display mode used for liquid crystal displays are classified into two large groups. One of them is an active/matrix mode in which a switching device is provided to each pixel. As an example of this mode, display mode using Thin Film Transistor (TFT) has been proposed. Whereas the display quality of this mode has reached a level which is comparable with that of Cathode Ray Tube (CRT), expansion of screen size is difficult and production cost is high. The other one is Super Twisted Nematic (STN) mode. Whereas this mode is improved in contrast and dependency on viewing angle compared with conventional simple matrix mode, display quality has not reached the level of CRT. However, production cost is low. Accordingly, these two modes have a merit and demerit when their quality and production cost are taken into account.
As a method which is expected to solve the problems of the two modes, ferroelectric liquid crystal (FLC) has appeared on the market. When "FLC" is simply remarked at present, it means Surface Stabilized Ferroelectric Liquid Crystal (SSFLC). The SSFLC was proposed by N. A. Clark and S. T. Lagerwall (Appl. Phys. Lett. 36, 899 (1980)). Since then, SSFLC is said to be a liquid crystal of the next generation; its development and commercialization is being worked by makers of household electric appliances and raw material manufactures; and improvements of its characteristics and its preparation for the market are also being conducted by them.
This is because ferroelectric liquid crystal display devices inherently have the following characteristics:
(1) Quick response PA1 (2) Memory-effect PA1 (3) Wide viewing angle
The characteristics mentioned above suggest a possibility of realizing display devices having a large capacity by using SSFLC, and the characteristics make SSFLC very attractive.
However, as the researches and developments advance, problems to be solved have been made clear. Among them, one of important subjects is development of a stable memory. As the cause of difficulty in stably developing memory, unevenness of smectic layer structure (for example, twisted alignment and chevron structure) and generation of internal inverse electric field which is considered to be caused by an excessively large spontaneous polarization are conceived.
As a means for developing stable memory-effect, a method has been proposed in which a ferroelectric liquid crystal composition having a negative dielectric anisotropy (hereinafter sometimes abbreviated to .DELTA..epsilon.) is used (c.f. Paris Liquid Crystal Conference, p. 217 (1984)). This method is called "AC stabilize effect". Liquid crystal molecules having a negative .DELTA..epsilon. have such a property that they turn to the direction parallel to that of glass substrates (major axis of the molecules points to the direction vertical to the direction of electric field) when an electric field is applied to the direction vertical to that of electrodes in a cell which is subjected to homogeneous aligning treatment. Spontaneous polarization responds to electric field when electric field of a low frequency is applied. Accordingly, when the direction of the electric field is reversed, liquid crystal molecules shift to another stable state as a consequence of its reversal, and come to the state parallel to the direction of substrates by the effect of .DELTA..epsilon.. When electric field of a high frequency is applied, spontaneous polarization becomes unable to follow the reversal of the electric field, only .DELTA..epsilon. has effects, shift of liquid crystal molecules does not occur even if the direction of electric field was reversed, and thus the liquid crystal molecules remain parallel to the substrates. This is a mechanism for developing memory-effect taking advantage of the AC stabilize effect. High contrast can be obtained by this mechanism, and its specific examples are already reported (c.f. SID '85 Digest p. 128 (1985)).
Separately, "Method for utilizing liquid crystal materials having a negative dielectric anisotropy" is proposed by P. W. H. Surguy et al., Ferroelectrics, 122, 63 (1991)). This is a promising method for actualizing a high contrast, and ferroelectric liquid crystal displays prepared by using this method are disclosed in P. W. Ross, Proc. SID, 217 (1992). These ferroelectric liquid crystal displays are described in detail in the followings.
In the case of ordinary ferroelectric liquid crystal materials dielectric anisotropy of which is not negative, .tau. (pulse width necessary to make it memorize) lowers in a monotone as voltage (V) increases. In contrast with, in the case of ferroelectric liquid crystal materials having a negative anisotropy, .tau.-V characteristic exhibiting a :minimum (relative minimum) value (.tau.-V.sub.min) is obtained. Surguy et al. have reported JOERS/Alvey driving method as a driving method which utilize the characteristic. Principle of this driving method is that the memory state of ferroelectric liquid crystal display devices is switched when a voltage of .vertline.Vs-Vd.vertline. is applied, and it is not switched when a voltage .vertline.Vs+Vd.vertline. which is higher than .vertline.Vs-Vd.vertline. is applied or when a voltage .vertline.Vd.vertline. which is lower than .vertline.Vs-Vd.vertline. is applied.
As described above, since ferroelectric liquid crystal materials having a negative dielectric anisotropy can be applied for display devices which utilize the AC stabilize effect and .tau..sub.min, they quietly have a possibility of being employed for actualizing ferroelectric liquid crystal display devices.
One of other important subjects posed by SSFLC is extremely high sensitivity of its optical response to temperature. In the case of TN type display mode, desired transmitting light intensity is obtained by synergism of dielectric anisotropy of liquid crystal molecules and electric field. Accordingly, the quantity of transmitting light to be obtained is nearly decided by dielectric constant of liquid crystal molecules and voltage to be applied, and viscosity affects only to the portions of transient optical response.
On the other hand, in the case of SSFLC, it has spontaneous polarization, and the quantity of transmitting light is changed by switching the stable state of liquid crystal molecules by using driving force PS-E produced by the spontaneous polarization (Ps) and electric field (E). Response speed (.tau.) at this time is approximately expressed by the equation EQU .tau..sup..varies. (.eta./PS.multidot.E) 1
and directly affected by viscosity (.eta.). Further, since the viscosity largely varies with temperature, response speed .tau. is liable to be affected by temperature. That is, in the case of SSFLC, in contrast to TN type display mode, it is directly affected by viscosity and becomes sensitive to temperature since desired quantity of transmitting light is obtained by taking advantage the transient optical response according to equation 1
This subjects relates to the principle for driving SSFLC. That is, it is extremely difficult to make the response speed of SSFLC independent from viscosity. On the other hand, it is very difficult to reduce the dependency of viscosity on temperature. Accordingly, in order to actualize display devices having a small dependency on temperature, spontaneous polarization Ps or electric field E which is a factor other than viscosity in equation 1 should be changed toward the direction in which the variation of viscosity with temperature is extinguished.
Whereas it is comparatively easy to change electric field E depending on the change in temperature, circuits become complex and IC drivers having a high withstand voltage become necessary in this case, leading to cost up of the devices. However, even if a function by which electric field E varies in response to the change in temperature is added to devices, it is possible to suppress the cost up by using liquid crystal compositions having a large dependency of spontaneous polarization Ps on temperature coping with the change in viscosity. That is, the use of such compositions having a large dependency of PS on temperature has such an advantage that the performances of liquid crystal display devices are improved.